1. Field of the Invention
The invention relates to a method for operating an automatic transmission as a function of, among other things, engine rotational speed. The invention also relates to an automatic transmission for a variable speed internal combustion engine, with a delivery system for a working medium with which at least one adjusting device is acted on by a first control device which, in turn, is controlled by a pilot pressure that can be accurately changed by a second control device to operate the adjusting device.
2. Description of the Related Art
A similar method and a similar automatic transmission are disclosed in German Patent Publication No. DE 195 46 293 A1, in which is disclosed a belt driven, conical pulley transmission with input side and output side pairs of conical disks. The power transmission between the two pairs of conical disks can be effected by a steel loop or band. Each pair of conical disks includes an axially movable disk and an axially fixed conical disk. The axially displaceable conical disk halves are arranged diagonally opposite each other. By an axial displacement of the movable conical disk halves, the effective lever arms of the pairs of conical disks become larger or smaller in opposite directions.
The axial displacement of the movable conical disk halves is achieved with an adjusting device. A first control device actuates the adjusting device. The first control device includes one or two hydraulic valves to which a pilot pressure is applied. The first control device is controlled by an electrically actuated second control device, for example a proportional valve. To adjust the transmission ratio of the belt driven, conical disk transmission, depending upon the demand, the pilot pressure is accurately changed by the second control device, for example by suitable software.
In the event of an electronic failure, the adjusting device can no longer be controlled by the first control device because of the failure of the electronically-operated second control device, and the automatic transmission will no longer function. Consequently, continued operation (limp home) of a motor vehicle with that type of automatic transmission is no longer possible by its own power, and it must therefore be towed away.
The object of the invention is to provide an appropriate method and an appropriate automatic transmission that enable continued operation by its own power when an electronic failure occurs and/or the second control device fails.
The object is attained with a method for operating an automatic transmission as a function of, among other things, the engine rotational speed, in that the transmission ratio is changed to a limp home condition of the automatic transmission after a starting procedure in order to maintain the engine rotational speed at a specific constant value. A transmission ratio of high speed results when the engine rotational speed increases, and a transmission ratio of low speed results when the engine rotational speed decreases. A constant engine rotational speed is maintained through the transmission ratio control in a limp home condition.
A preferred embodiment of the method is characterized in that the transmission ratio is adjusted to a high speed ratio, after the starting process, at increasing engine rotational speed, until the minimum possible transmission ratio condition is reached, which is then maintained during further increases in engine rotational speed. After reaching the minimum transmission ratio condition, the engine rotational speed can no longer be maintained constant, and it increases until the engine performance is in equilibrium with the running resistance.
Another preferred embodiment of the method is characterized in that the transmission ratio, after the starting process, is adjusted to a low speed ratio at decreasing engine rotational speed, until the largest possible transmission ratio condition is reached, which is then maintained during further decreases in engine rotational speed. After reaching the largest possible transmission ratio condition, the engine rotational speed can no longer be maintained constant and decreases.
A further preferred embodiment of the method is characterized in that, when the rotational speed is increased further, the transfer of torque from an input element to an output element is interrupted with the aid of a clutch or converter. The engine is thereby prevented from stalling.
A further preferred embodiment of the method is characterized in that the specific value at which the engine rotational speed is maintained constant is greater than the so-called stall speed. The maximum rotational speed at which a clutch or a converter can still slip is designated as the stall speed. The stall speed is therefore the rotational speed at which the converter or the clutch stops the engine with completely applied brakes and full throttle. A certain minimum difference between the stall speed and the transmission ratio control rotational speed is to be maintained for safety reasons.
The object is achieved in an automatic transmission for a variable rotational speed internal combustion engine, with a delivery system for a working medium, with which at least one adjusting device is actuated by a first control device, which, in turn, is controlled by a pilot pressure that can be specifically changed by a second control device in order to operate the adjusting device, in that in addition to the second control device a third control device is provided, which is actuated by a working medium transported by the delivery system, with the help of which the pilot pressure for controlling the first control device is changed as a function of the rotational speed of the internal combustion engine, when a switching device is switched from a normal position to a limp home position.
The automatic transmission in accordance with the invention is distinguished by an additional third control device, which is hydraulically or pneumatically operated by the working medium transported by the delivery system. As a result, the operation of the adjusting device is ensured even when the second control device fails. In that way an emergency operation of the automatic transmission is ensured, so that continued operation, for example to the nearest service station, is possible.
One embodiment of the automatic transmission is characterized in that the delivery system for the working medium is composed of a pump whose delivered volumetric flow depends upon the rotational speed of the internal combustion engine. That can be achieved, for example, by coupling the input of the pump with the camshaft or the crankshaft of the internal combustion engine. The volumetric flow of the pump, which is dependent upon the speed of the internal combustion engine, is utilized in a so-called limp home condition of the automatic transmission for controlling the transmission ratio of the transmission and/or a starting clutch.
A further embodiment of the automatic transmission is characterized in that the third control device has an axially-displaceable, spring-biased control plunger with a first and second working surface for the working medium delivered by the delivery system, as well as with a third working surface, which is contacted by the regulated pilot pressure, whereby the first and second working surfaces on the control plunger of the third control device, which are contacted by the working medium, are of the same size. In the normal condition of the transmission, the same working medium pressure acts on the first and second working surfaces on the control plunger of the third control device. The magnitude of the regulated pilot pressure can be adjusted by the spring bias acting on the control plunger. The spring-biased control plunger of the third control device functions as a pressure regulator that maintains the pilot pressure at a constant value. Two control edges can be formed on the control plunger, which ensure that the pilot pressure will decrease when it exceeds a specified value, and which will ensure that the pilot pressure will increase when it drops below the specified value.
A further embodiment of the automatic transmission is characterized in that the first and second working surfaces, for the working medium, of the control plunger of the third control device are connected to each other through a connecting conduit, in which an orifice plate is provided, through which flows the working medium that is delivered by the delivery system when the switching device is in the limp home position. In the normal condition of the automatic transmission, the working medium delivered by the delivery system does not flow through the orifice plate. In the limp home condition, the switching device ensures that the working medium delivered by the delivery system flows through the orifice plate. That leads to a different working medium pressure acting on the first working surface of the control plunger of the third control device than on the second working surface. The pressure difference increases with the volumetric flow delivered by the delivery system, which, in turn, is dependent upon the rotational speed of the connected internal combustion engine. The pilot pressure that exists at the third working surface of the control plunger is changed by the pressure difference between the first and second working surfaces for the working medium at the control plunger of the third control device, which is dependent upon the rotational speed of the internal combustion engine. A throttle can be utilized instead of the orifice plate.
A particular embodiment of the automatic transmission is characterized in that the switching device includes an axially displaceable, spring-biased switching plunger, which is moved from its normal position into its limp home position during a limp home condition of the automatic transmission by the spring biasing force, whereby a direct connection between the delivery system and the adjusting device is interrupted and a connection between the delivery system and the adjusting device is released by the orifice plate in the connecting conduit between the first and second working surfaces, for the working medium, that are on the control plunger of the third control device. By the spring biasing force acting on the switching plunger, the switching plunger is also automatically shifted in a simple way into its limp home position when there is a total failure of the electronics. Simultaneously, by the displacement of the plunger to its limp home position, the flow through the orifice plate is forced into the connecting conduit between the first and second working surfaces on the control plunger of the third control device.
A further particular embodiment of the automatic transmission is characterized in that a device for adjusting the transmission ratio of the automatic transmission and a device for adjusting the starting clutch are coupled with the delivery system as well as with the third control device. That provides the advantage that only one delivery system is required to ensure a sufficient transmission ratio as well as a satisfactory functioning of the starting clutch in a limp home condition of the transmission. If a first control device with an increasing characteristic curve is utilized for the control of the adjusting device of the starting clutch, a reversing slide valve is required to ensure a proper function of the starting clutch during limp home.
A further particular embodiment of the automatic transmission is characterized in that in the limp home position of the switching device, between the third pilot pressure working surface on the control plunger of the third control device and a connection to a pressure relief chamber, a control pressure connecting conduit is disconnected, in which there are arranged, starting from the connection to the pressure relief chamber, a first orifice plate, a branch to the adjusting device for the starting clutch, a second orifice plate, and a branch to the transmission ratio adjusting device. The cascade-like arrangement of the orifice plates makes possible agreement between rotational speed and starting pressure, between rotational speed and adjusting pressure, as well as between starting pressure and adjusting pressure. In the limp home position of the switching device, the pilot pressure drops to almost zero at high engine rotational speed. Consequently, the starting clutch is engaged and a transmission ratio of fast (overdrive) results. With decreasing engine rotational speed, the pilot pressure increases and a transmission ratio of slow (underdrive) results.
The stepless automatic transmission in accordance with the invention can also be called a CVT (continuously variable transmission). In a CVT, among other things, the starting and transmission ratio adjustment functions must be ensured. The starting function is assured either directly through a starting clutch or through a torque converter, after an accompanying reverse set clutch has been engaged. The starting clutch is ordinarily engaged by pressure. That can occur, for example, with a valve that increases or reverses the pressure without current. A torque converter starts practically by itself, without hydraulic control contact.
The transmission ratio of the automatic transmission must always be capable of being adjusted, because without pressure and without additional hydraulic transmission ratio adjustment, a transmission ratio of close to 1:1 (somewhere between third and fourth gear) would be set. Therefore, a transmission ratio adjustment is absolutely required in the limp home condition. As a rule, for safety reasons, the transmission ratio adjustment is designed in such a way that when the pilot pressure is close to zero an adjustment of the transmission ratio to fast results.
The basic idea of the emergency operation is as follows. The starting clutch is engaged hydraulically or it is started by the torque converter. Only when that process has been completed does the transmission ratio control to a constant engine rotational speed begin. That means that the transmission ratio control operates in the emergency operating mode so that a constant engine rotational speed is regulated. The engine rotational speed can be, for example, 3,500 revolutions/minute. When the engine rotational speed increases, the transmission ratio is automatically set to fast. When the engine rotational speed decreases, the transmission ratio is automatically set to slow. When a further adjustment of the transmission ratio to fast is no longer possible, the longest transmission ratio is maintained, even when the engine rotational speed increases further. When further adjustment of the transmission ratio to slow is no longer possible, the engine rotational speed drops further. It is only then that the starting clutch or the converter disengages.
During a power failure, the switching device switches to the limp home position. The pilot pressure of the first control device is uncoupled from the second control device and is controlled by the third control device. On the basis of the production by a metering orifice of a pressure that is proportional to the engine rotational speed, the pilot pressure on the first control device is decreased when the engine rotational speed, and thereby the pressure on the metering orifice plate, increases. That, in turn, causes an adjustment of the transmission ratio to fast, and the opposite to slow. The engine rotational speed that is to be controlled results from a coordination of the size of the metering orifice with the springs of the first control device. The connection between increasing pressure on the metering orifice plate and decreasing pilot pressure can be realized with a separate pressure reducing valve.